The rocker arm serves for opening and closing a valve which is provided on a cylinder head of an engine of an automobile or the like, and has a pair of side wall portions opposed to each other, a connection portion for a valve stem receiving portion which interconnects inner side surfaces of the pair of side wall portions at one end in a longitudinal direction, a connection portion for a pivot receiving portion which interconnects inner side surfaces of the pair of side wall portions at the other end in the longitudinal direction, and an open portion for arranging a roller which is formed between the connection portion for the valve stem receiving portion and the connection portion for the pivot receiving portion. Specifically, the rocker arm has such a complicated three-dimensional shape that the pair of side wall portions and the connection portion for the valve stem receiving portion form a substantially H-shape in section, the pair of side wall portions and the connection portion for the pivot receiving portion form a substantially U-shape in section, and the pair of side wall portions interposing the open portion for arranging the roller form a substantially II-shape in section.
In this rocker arm, a valve stem of an engine comes into contact with the valve stem receiving portion which is formed in the connection portion for the valve stem receiving portion, a pivot of a lash adjuster of the engine comes into contact with the pivot receiving portion which is formed in the connection portion for the pivot receiving portion, and a cam of the engine slides along the roller which is rotatably provided in the open portion for arranging the roller.
Recently, for the purpose of complying with requests for energy saving, high speed rotation of the engine, and cost cutting, it has been endeavored to make the rocker arm lightweight and rigid, and to develop a new process for producing the rocker arm. The inventor and his group have successfully developed a production process in which the complicated three-dimensional shape as described above can be formed only by plastically deforming a piece of metal sheet, and a metal flow can be continued between the pair of side wall portions and the connection portion for the valve stem receiving portion (within the substantially H-shaped section), and have disclosed this process (Refer to Patent Document 1, for example).                Patent Document 1: Japanese Patent Publication No. JP-A-2004-25240 (Page 4, FIG. 8)        